lkml.org 
[lkml]   [2007]   [Jun]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2.6.21] cramfs: add cramfs Linear XIP
Nick Piggin wrote:
> Carsten Otte wrote:
>> The current xip stack relies on having struct page behind the memory
>> segment. This causes few impact on memory management, but occupies
>> some more memory. The cramfs patch chose to modify copy on write in
>> order to deal with vmas that don't have struct page behind.
>> So far, Hugh and Linus have shown strong opposition against copy on
>> write with no struct page behind. If this implementation is acceptable
>> to the them, it seems preferable to me over wasting memory. The xip
>> stack should be modified to use this vma flag in that case.
>
> I would rather not :P
>
> We can copy on write without a struct page behind the source today, no?
> What is insufficient for the XIP code with the current COW?

I've looked at the -mm version of mm/memory.c today, with intend to
try out VM_PFNMAP for our xip mappings and replace nopage() with fault().
The thing is, I believe it does'nt work for us:
* The way we recognize those mappings is through the rules set up
* by "remap_pfn_range()": the vma will have the VM_PFNMAP bit set,
* and the vm_pgoff will point to the first PFN mapped: thus every
* page that is a raw mapping will always honor the rule
*
* pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >>
PAGE_SHIFT)

This is, as far as I can tell, not true for our xip mappings. Ext2 may
spread the physical pages behind a given file all over its media. That
means, that the pfns of the pages that form a vma may be more or less
random rather than contiguous. The common memory management code
cannot tell whether or not a given page has been COW'ed.
Did I miss something?

so long,
Carsten
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-06-15 15:57    [W:0.291 / U:0.248 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site